A standard door latch for a motor vehicle includes a housing mounted on the door edge and formed with a recess into which is engageable a bolt projecting from the respective door post. A fork is pivotal in the housing between a latching position engaging around the bolt and holding it solidly in the recess and a freeing position permitting the bolt to enter and leave the recess. A latch pawl engageable with the fork can hold it in the latched position. The latch pawl in turn is typically spring loaded and can be controlled via appropriate levers both from an inside door handle and an outside door handle, either of which can therefore operate the latch to allow the door to be opened.
In addition the door can be locked by means of a button or lever inside the door and a key cylinder outside the door, and also frequently by an actuator operated by a central lock system. When locked the outside door handle is either impeded from moving so it cannot move the latch pawl, or it is decoupled from the latch pawl so its actuation is ineffective.
In a standard lock system the inner door handle and inner lock button or lever remain effective even when the door is locked so that the outside handle does not work. In an antitheft system, however, there is, in addition to the locked and unlocked conditions of the latch, an antitheft mode or position in which the latch cannot be opened by either of the door handles and in which the inside lock element is also ineffective to unlock the door. Thus when in the antitheft position a would-be thief cannot even open the door by forcing the window and operating the door from inside.
As described in U.S. Pat. Nos. 4,342,209 of F. Kleefeldt and 4,669,283 of J. Ingehoven the antitheft position is set by an electric-motor actuator having a motor whose output shaft is a spindle on which is threaded a nut that acts as a detent that physically impedes and latches the inside lock element in the locked position. Thus the structure must be strong enough to withstand whatever force the would-be thief is willing to apply. Hence the elements must be made fairly robust and, therefore, are of expensive manufacture.
Another substantial disadvantage of the known antitheft systems is that the antitheft position is set by an actuator, typically an electric motor, and the latch can only be reset out of this antitheft position by means of this motor. Thus if the vehicle's electric power fails it becomes fairly impossible to enter the vehicle, even for someone with the right key that typically can operate the main central-lock switch. Similarly if the connection to a one of the latch actuators fails or one of the electric motors burns out, the respective latch can be left stuck in the antitheft position and painstaking disassembly of the door is required to effect the necessary repair.